As representative apparatuses among microwave heating devices for performing heating processing on objects through microwaves, there have been microwave ovens. A microwave oven is adapted to radiate microwaves generated from a microwave generator to the inside of a metallic heating chamber, thereby causing an object to be heated within the heating chamber to be subjected to dielectric heating through radiated microwaves.
Conventional microwave ovens have employed magnetrons as such microwave generators. Such a magnetron generates microwaves, which are radiated to the inside of the heating chamber through a waveguide tube. A non-uniform microwave electromagnetic-field distribution (microwave distribution) within the heating chamber causes that uniform microwave heating for the object cannot be performed.
As means for uniformly heating an object to be heated within a heating chamber, there is a mechanism adapted to rotate a table on which an object to be heated is placed so as to rotate the object to be heated, a mechanism adapted to rotate an antenna which radiates microwaves while fixing the object to be heated, or a mechanism adapted to shift phases of microwaves from microwave generator using a phase shifter. It is a general method for heating uniformly to an object that the object to be heated is heated with changing directions of the microwaves radiated to the object by using any driving mechanism as mentioned above.
On the other hand, in order to constitute simply, a method of carrying out uniform heating without having drive mechanism is demanded, and the method of using a circular polarization of which a polarization plane of electric field rotates in time is proposed. Since dielectric heating is carried out on the basis of the principle that to-be-heated an object having dielectric loss is heated with the electric field of microwave, it is thought that using the circular polarization of which an electric field rotates has an effect in equalization of heating.
As concrete way for generating the circular polarization, for example, as shown in FIG. 12, U.S. Pat. No. 4,301,347 (Patent Literature 1) discloses a structure using a circular polarization opening 1202 of an X shape which is formed to have a crossing shape on a waveguide tube 1200. Also, Japanese Patent No. 3510523 (Patent Literature 2) discloses a structure which arranges two openings 1301 of rectangular slits to be extended in a direction perpendicular on a waveguide tube 1300, and the openings 1301 are arranged to have an interval apart from each other, as shown in FIG. 13. Furthermore, Unexamined Japanese Patent Publication No. 2005-235772 (Patent literature 3) discloses a patch antenna 1401 which is connected to waveguide tube 1400 for propagating microwaves from a magnetron 1404, as shown in FIG. 14. The patch antenna 1401 is configured to generate a circular polarization with cut portions 1402 which are formed on a plane of the patch antenna 1401.
For example, some conventional microwave heating devices have been structured to have a rotatable antenna and an antenna shaft which are arranged within a waveguide tube and, further, to drive a magnetron while rotating this antenna through a motor, thereby alleviating the non-uniformity in the microwave distribution within the heating chamber.
Further, Unexamined Japanese Patent Publication No. S 62-64093 (Patent Literature 4) suggests a microwave heating device which is provided with a rotatable antenna at a lower portion of a magnetron and is adapted to direct air flows from a blower fan to the blades of this antenna for rotating the antenna by the wind power from the blower fan, in order to change the microwave distribution within the heating chamber.
As an example of provision of such a phase shifter, Patent Literature 1 describes the microwave heating device which is adapted to alleviate heating unevenness in an object to be heated through microwave heating and to reduce a space of feeding portions. This Patent Literature 1 suggests the microwave heating device having a rotary phase shifter 1201 and a single microwave radiating portion 1202 for radiating circularly-polarized waves within the heating chamber, as shown in FIG. 12.